linux-kernel - Re: [PATCH v3] wait_on_bit: add an acquire memory barrier

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Message-ID: <alpine.LRH.2.02.2208261344480.28812@file01.intranet.prod.int.rdu2.redhat.com>
Date:   Fri, 26 Aug 2022 13:50:51 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH v3] wait_on_bit: add an acquire memory barrier



On Fri, 26 Aug 2022, Linus Torvalds wrote:

> That code sequence really doesn't matter, but it was interesting
> seeing the generated code. Not pretty, but the ugliest part was
> actually how the might_sleep() calls in those helper functions result
> in __cond_resched() when PREEMPT_VOLUNTARY is on, and how horrid that
> is for register allocation.

Perhaps, use __attribute__((no_caller_saved_registers)) on 
__cond_resched() ?

Mikulas